A particle counting apparatus of this kind, as disclosed in Patent Literature 1, provides a measurement targeted gas containing sample particles to flow in a flow path in which a working liquid (alcohol such as ethanol, isopropanol, and butanol) is diffused in a state of saturated vapor and the working liquid is thereafter condensed by cooling with the sample particles as cores to produce droplets of the working liquid so that the number of the sample particles is counted by counting the droplets of the working Liquid to thereby measure a concentration thereof.
In a conventional construction of a particle counting apparatus as shown in e.g. FIG. 9, however, in order to fill the flow path with the saturated vapor of the working liquid, a porous member including the flow path vertically passed therethrough is inserted into a cylindrical housing to be fitted, and lower end portions of the porous member are immersed in the working liquid so that the working liquid is impregnated into the entire part of the porous member, and therefore the flow path penetrating the porous member is brought into the condition of being saturated with the vapor of the working liquid.